Letterman: I had always wanted to do a comedy, kind of like the film The Dirty Dozen. Oddly enough it turned out to be a way to do my ‘Dirty Dozen’ comedy, as the monsters are a type of rogue team that goes up against aliens invading the Earth. So I loved the idea.

What was it like working with co-director Conrad Vernon?

Letterman: “Conrad’s really great, a talented storyboard artist and director and a voice talent as well – I mean he’s the voice of the Gingerbread Man [from the Shrek movies].

It was a great advantage because he could do all the actors’ voices – he impersonates every single person in the cast. So while we were developing the story, we could build the movie while we were waiting for our chance with the actors.

I mean, that was just one wonderful side benefit. But he’s great, and we really bounce off each other well.

How did you feel about sharing the Monsters Vs Aliens chair?

Vernon: From the beginning, we didn’t try to delineate jobs, but rather to create a back-and-forth way of working. We were in constant contact, pitching ideas about scenes and characters to each other. Our goal all along was to create a cohesive and entertaining film, and we did that by keeping each other in the loop.

It assured we both stayed on the same page about every aspect of the film, and we weren’t off separately making two different projects. Always being clear about what film we were making – that kept it on track.

How did Reese Witherspoon come to be the voice of lead character Susan?

Letterman: The first time Reese Witherspoon came in and we actually met her, Jeffrey brought her by the studio and gave her a tour – he was basically showing her all the movies going on to see what she would respond to.

Conrad and I were waiting and she came in and we gave her a basic pitch of the idea of the movie and explained to her that we really wanted to make the female lead the hero character to balance out the dudes in the film. When she heard that her eyes just lit up.

Vernon: Reese connected to the story and the character of Susan. She e-mailed Jeffrey the next day and said she wanted to come onboard.

What was it that made Witherspoon perfect for the role?

Vernon: We thought it would be great and a nice contrast to have a petite girl play a giant woman, and we had fun with that.

Susan is living vicariously through her fiancé. We liked taking that idea and turning it around, so when she becomes the most powerful woman in the world, she realises that she doesn’t really need him to lead the amazing life that she has always wanted.

What is your reasoning for casting Hugh Laurie as a cockroach?

Letterman: We wanted someone to be very formal and erudite, because we thought that would be a great contrast with the whole ‘cockroach bad’ thing. And so we immediately thought of Hugh.

A lot of people don’t realise, at least people in the States, that he’s a Brit.

Hugh Laurie is mostly known in the States for House, but he is obviously a famous actor in the UK, not least for his role in The Black Adder [TV series] with Rowan Atkinson. He is an incredible comedic actor. We were fans prior to House, so it was fortunate to have him on the movie.

Vernon: When we met him, he had this look on his face, like, ‘why are you guys talking to me right now?’

We toured him around the DreamWorks Animation campus, and wound up in the editing room, showing him storyboard sequences of the film. We talked casually and really hit it off. He said ‘Yes’ on the spot. Once he got in the booth, he did not disappoint.